Field of the Invention
The present invention relates to a hub clutch for an automotive vehicle suitable for use in a part-time 4WD (four wheel drive) vehicle which can be driven in both 4WD travel mode and 2WD (two wheel drive) travel mode.
Description of the Prior Art
In a 2-4WD selectable vehicle (referred to as part-time 4WD vehicle), when the vehicle is driven in the 2WD travel mode, since two front or rear wheels are disconnected from an engine, a driving mechanism including a propeller shaft, a differential shaft, etc. on the disconnected side is driven by the rotating wheels, so that there exist various problems in that noise and vibration are generated; the fuel consumption rate increases; the lifetime of the driving mechanism decreases, etc.. To overcome these problems, a hub clutch is used to disconnect the non-driven vehicle wheels from the non-driven driving mechanism. An example of these hub clutch (an apparatus for controlling the disconnection operation between the wheels and the driving mechanism) is disclosed in U.S. Pat. No. 4,627,512 entitled AIR ACTUATED CLUTCH FOR FOUR WHEEL DRIVE VEHICLES, as shown in FIG. 1. In the drawing, this apparatus is composed of a hub lock member 203 rotatable together with a vehicle wheel 201; a driving tooth ring member 207 rotatable together with a wheel drive shaft 205; a clutch ring 209 for connecting (locking) or disconnecting (releasing) these two members 203 and 207, when shifted; an air tight chamber 213 for shifting the clutch ring 209 against an elastic force of a spring 208 by pressure supplied from the pressure source 211; a vehicle body side spindle 215 through which a wheel drive shaft 205 is passed for spline engagement; and some sealing members 217, 219, etc. disposed between the hub lock member 203 and the wheel drive shaft 205 to keep the airtight chamber 213 under airtight condition; etc.. In 4WD travel mode, the hub lock member 203 is locked to the driving tooth ring member 207 via the clutch ring 209 to drive the vehicle wheel 201 via the hub lock member 203; and in 2WD travel mode, the hub lock member 203 is released from the driving tooth ring member 207 via the clutch ring 209 to disconnect the wheel drive shaft 205 from the vehicle wheel 201.
In the prior-art apparatus, it is required to increase the contact pressure of these sealing members 217 and 219 disposed between the rotary members 203 and 205 and the fixed member against the sliding surfaces in order to maintain a high sealing capability. However, when the contact pressure is increased, there exists a problem in that the rotative resistance of the rotary members 203 and 205 increases. In addition, when the contact pressure of the sealing members is increased, since the sliding surfaces thereof tend to be easily worn away and therefore the sealing lifetime is reduced, there arises another problem in that pressurized air easily leaks through these sealing members 217 and 219 in the pneumatic system composed of the pressure source 211, a pressurized air supply passage 221, the airtight chamber 213, etc.. In case of leakage in the pneumatic system, the apparatus becomes inoperative. That is, where the locking function is disabled, the vehicle cannot be driven in the 4WD travel mode; and where the unlocking function is disabled, the vehicle cannot be driven in the 2WD travel mode.